Laser-acoustic joint detection technology is an emerging technology in the field of space-underwater communication, underwater target detection and ocean monitoring in the marine environment. It plays an important role in many new high-end marine equipment manufacturing, deep-sea exploration and security fields. However, accurate detection of multiple sound sources in the case of spectrum aliasing of detection signals has been a technical bottleneck. The purpose of this paper is to extract the underwater sound field information from the sound waves on the water surface, and demodulate the sound frequency of the sound sources close to the underwater frequency. According to the modulation theory of incident laser on the surface of water, this paper introduces the basic principle of laser interferometry to detect sound waves on water surface. This paper proposes a method for detecting the frequency of underwater acoustic signals using optical heterodyne. The expression of the photodetector output current is derived under a plurality of underwater sound source signals. The time domain and frequency domain characteristics of the detected surface wave interference signals are analyzed by simulation. And the feasibility of the method was verified. The results show that the method can detect the frequency and amplitude of the ideal surface wave. In order to obtain a more accurate audible frequency of underwater sound source, this paper proposes a frequency demodulation method based on Hilbert transform. And specific mathematical expressions are derived. This solves the frequency demodulation problem of spectral aliasing of the coherent detection signal. It provides a new method for the detection and processing of underwater acoustic signals.
Proc. SPIE. 11436, 2019 International Conference on Optical Instruments and Technology: Optical Sensors and Applications
KEYWORDS: Optical fibers, Signal attenuation, Calibration, Demodulation, Raman spectroscopy, Signal processing, Temperature sensors, Raman scattering, Temperature metrology, High temperature raman spectroscopy
With the rapid development of modern society, the advent of the era of big data makes the exchange of information increasingly frequent and important. The distributed fiber Raman temperature measurement system is a brand-new sensing technology that has rapidly developed in recent years. As a transmission medium, it uses spontaneous Raman scattering to acquire back Raman scattering signals through a high-speed acquisition card. Since this signal carries temperature information, this signal is amplified and denoised and then demodulated to obtain a curve with temperature information. In this paper, we study the optical fiber temperature sensing scheme based on Raman scattering and analyze its working principle. By analyzing and comparing the demodulation method of sensor temperature, a method of demodulating temperature at different temperatures by using anti-Stokes fiber temperature as a reference channel is proposed. The relationship between the Raman ratio and the distance is demodulated. Because the traditional calibration scheme fails to take the environmental temperature value into account, this paper adopts a novel dynamic multi-segment fiber temperature calibration method, and verifies the feasibility of the calibration scheme through experiments. The result shows that with the change of the external environment temperature, the temperature of the sensing fiber can be accurately demodulated, the temperature demodulation result is more accurate, the measurement error is less than 1°C. The resulting system is more stable and can adapt to complex environmental changes. Since the light will become soft under high temperature conditions, this paper calculates the relationship between fiber loss coefficient and temperature. It is found that the continuous summation method can better solve the loss problem, thereby effectively improving the system signal-to-noise ratio.